I. Field
The present disclosure relates generally to communication, and more specifically to techniques for reducing peak-to-average power ratio (PAPR) of a transmission in a wireless communication system.
II. Background
Wireless communication systems are widely deployed to provide various communication content such as voice, video, packet data, messaging, broadcast, etc. These wireless systems may be multiple-access systems capable of supporting multiple users by sharing the available system resources. Examples of such multiple-access systems include Code Division Multiple Access (CDMA) systems, Time Division Multiple Access (TDMA) systems, Frequency Division Multiple Access (FDMA) systems, Orthogonal FDMA (OFDMA) systems, and Single-Carrier FDMA (SC-FDMA) systems.
A wireless communication system may support data transmission on multiple subcarriers. These multiple subcarriers may be obtained with orthogonal frequency division multiplexing (OFDM), single-carrier frequency division multiplexing (SC-FDM), or some other multi-carrier modulation scheme. OFDM and SC-FDM partition the system bandwidth into multiple (K) orthogonal subcarriers, which are also commonly referred to as tones, bins, etc. Each subcarrier may be modulated with data. In general, modulation symbols are sent in the frequency domain with OFDM and in the time domain with SC-FDM.
Multi-carrier modulation schemes such as OFDM and SC-FDM may have certain desirable characteristics, such as the ability to combat multipath effects. However, a major drawback with OFDM, and to a lesser extend with SC-FDM, is a high PAPR of an output waveform, which means that the ratio of the peak power to the average power of the output waveform can be high. For OFDM, the subcarriers may be independently modulated with data, and high PAPR may result from possible in-phase addition of all of the subcarriers when they are independently modulated with data. In fact, it can be shown that the peak power may be up to Q times greater than the average power for OFDM, where Q is the number of subcarriers used for transmission.
A high PAPR of an output waveform generated with multi-carrier modulation normally requires a power amplifier to be operated at an average power level that may be much lower than the peak power level, i.e., backed off from peak power. This is because large peaks in the output waveform may cause the power amplifier to operate in a highly non-linear region or possibly clip, which may then cause intermodulation distortion and other artifacts that may degrade performance. By operating the power amplifier at a back-off from peak power, the power amplifier can handle large peaks in the output waveform without generating excessive distortion. However, the back-off represents inefficient operation of the power amplifier during other times when large peaks are not present in the output waveform. It may thus be highly desirable to reduce the PAPR of the output waveform.